Cylindrical rolls are utilized in a number of industrial applications, especially those relating to papermaking. Such rolls are typically employed in demanding environments in which they can be exposed to high dynamic loads and temperatures and aggressive or corrosive chemical agents. As an example, in a typical paper mill, rolls are used not only for transporting a fibrous web sheet between processing stations, but also, in the case of press section and calender rolls, for processing the web sheet itself into paper.
Typically rolls used in papermaking are constructed with the location within the papermaking machine in mind, as rolls residing in different positions within the papermaking machines are required to perform different functions. Because papermaking rolls can have many different performance demands, and because replacing an entire metallic roll can be quite expensive, many papermaking rolls include a polymeric cover that surrounds the circumferential surface of a metallic core. By varying the polymer or elastomer employed in the cover, the cover designer can provide the roll with different performance characteristics as the papermaking application demands. Also, repair, regrinding or replacement of a cover over a metallic roll can be considerably less expensive than the replacement of an entire metallic roll.
In many instances, the roll cover will include at least two distinct layers: a base layer that overlies the core and provides a bond thereto; and a topstock layer that overlies and bonds to the base layer and serves the outer surface of the roll (some rolls will also include an intermediate “tie-in” layer sandwiched by the base and top stock layers). The layers for these materials are typically selected to provide the cover with a prescribed set of physical properties for operation. These can include the requisite strength, elastic modulus, and resistance to elevated temperature, water and harsh chemicals to withstand the papermaking environment. In addition, covers are typically designed to have a predetermined surface hardness that is appropriate for the process they are to perform, and they typically require that the paper sheet “release” from the cover without damage to the paper sheet. Also, in order to be economical, the cover should be abrasion- and wear-resistant.
In some instances, circumferential grooves are included in the roll cover to encourage drainage from the paper sheet. Improved drainage typically results in energy savings for the paper mill. Grooves are typically formed in harder rolls (10 P&J or lower) and have a width of between about 0.020″ and 0.040″ and a depth of between about 0.090″ and 0.125.″ Generally speaking, deeper grooves are preferred due to the increased drainage they can encourage. The configuration of a typical set of grooves for harder covers is shown in FIG. 1, wherein a section of a cover 10′ with grooves 30′ is illustrated. Covers formed of softer materials (e.g., a rubber having a hardness of 35 on the Pusey and James (P&J) scale) have not included grooves, because the walls of the grooves tended to deflect toward each other under load, thereby pinching off the grooves and preventing water from entering the grooves as they passed through the nip. In particular, the high Poisson's ratio of rubber caused grooves formed in softer rubber materials to deform and collapse.
Tissue manufacture typically employs soft covers in order to conform the paper sheet to a mating structure (sometimes called a “Yankee can”) for improved drying efficiency. However, grooves of the type described above have not been used in soft covers employed in tissue production for the reason stated above: namely, that the walls of the grooves deform and close the groove, thereby reducing or eliminating the benefits of the grooves.